The present invention is generally related to radio receivers and more particularly to a digital radio receiver having an adaptive bandwidth receive channel filter which is varied in order that interference, as determined by bit error rate and signal strength measurements, be minimized.
Radio receivers are inherently designed to receive signals from a desired signal source which transmits on one radio frequency and to reject signals from an undesired signal source which transmits on another radio frequency. One element which enables such discrimination between a desired signal and an undesired interfering signal is the receive channel filtering. In conventional receivers, a relatively narrow bandwidth bandpass filter is centered in frequency around the desired signal frequency (or the frequency to which the desired signal frequency is converted in a superheterodyne receiver) to pass the desired signal while rejecting signals on adjacent and other channels.
It has been a common practice in communications receiver design to provide a receiver channel filter or filters which are switchably variable in bandwidth and which may be variable in center frequency. The user of such a communications receiver can manually vary the receive channel filter bandwidth to accommodate the spectrum of signal to be received while avoiding interference from an adjacent channel. Additionally, users have been able to switch receiving bands in consumer receiver products and television receivers, thereby producing the selection of different filters or different filter bandwidths.
Commercial two-way transceivers (and some citizen's band transceivers) have employed in the receiver a noise reduction circuit, often called a noise blanker, which detects noise at a frequency separated from the frequency of the desired signal and briefly mutes the receiver during noise bursts. Such circuits, however, do not vary the received channel bandwidth and do not provide protection against adjacent channel interference.
Cellular radiotelephone transceivers employed by radiotelephone subscribers in analog cellular systems have adequate protection against adjacent channel interference provided by both the receive channel filter and a geographic channel allocation plan which places adjacent channels at a distance from each other. In some applications, radiotelephone service may be provided from two systems having different radio characteristics. U.S. Pat. No. 4,972,455 discusses such a radiotelephone. The digital cellular system proposed for use in the United States, employs a digital modulation which uses, on the average, a wider portion of the allocated 30 KHz channel bandwidth than the analog cellular modulation. This fuller channel occupation reduces the margin of protection from adjacent channel interference and in some cases results in objectionable interference being heard by the digital cellular radiotelephone user.